Modelling the impact of various future scenarios on unmet water demand using Water Evaluation and Planning Model (A Case Study Of Nairobi City County)

Abstract

Nairobi city is currently experiencing both physical and economic water shortage challenges, which are caused by the city's rapid population growth, improved living conditions, dilapidated water infrastructure, and exacerbated by severe climate change impact. The management of the water shortage crisis in the city has primarily been on physical infrastructure development as opposed to the holistic water resource management approach. The main objective of this study was to apply the Water Evaluation and Planning (WEAP) model to analyse the current unmet water demand and to evaluate the effect of various future scenarios on the unmet water demand of Nairobi City. The specific objectives were; To evaluate the existing water supply and demand based on the current situation using the WEAP model; To predict the impact of population growth, improved living standards, and Non-Revenue Water (NRW) scenarios on future water demand and supply options of the city using WEAP model; and To assess the impact of introducing Rain Water Harvesting (RWH) on the unmet water demand of the city using WEAP model. The meteorological, water supply, and demand data obtained from the Kenya Metrological Department and the Nairobi City Water and Sewerage Company respectively were used to set up the model. The Soil Moisture Method embedded in the WEAP model and the monthly variation methods were used to simulate the catchment runoff and city demand respectively. The observed volume data for the Thika Dam from the year 1997 to 2016 was used for both the calibration and validation of the model. During the calibration and validation, four quantitative statistical parameters were used to check the performance of the model to represent the catchment. The coefficient of determination (R2 ), Nash Sutcliffe (NSE), Standard deviation of measured data (RSR), and the Percentage Bias (PBIAS) values were 0.70, 0.98, 0.13, and 4.9 respectively for calibration and 0.74, 0.98, 0.15, and 9.1 for validation. After the model setup, a reference scenario was created to represent the current water supply system and project it to the study period of 2021 to 2040. Future Scenarios were then built from the reference scenario. Two socioeconomic transformation scenarios were developed to examine the impact of rapid population increase and rising standards of living. The results in the reference scenario showed that the city would only satisfy 19% of its water needs by 2040 with an unmet water demand of 948 million cubic meters. The population growth will increase the unmet water demand to 1,036 million cubic meters by 2040, while the improved living standard will increase it to 1,309 million cubic meters in the same year. On the contrary, the reduction of non-revenue water and the introduction of rainwater harvesting will have a significant positive impact on the unmet water demand. The unmet water demand will be 624 million cubic meters and 822 million cubic meters by 2040 for non-revenue water reduction and rainwater harvesting scenarios respectively. Based on these results, it was concluded that the WEAP model can assist the water utility in making decisions that will improve water service delivery within the city. The water utility should adopt rigorous non-revenue water reduction strategies while the county government should put in place relevant legislation to operationalize rainwater harvestin